This situation is obtainable with semiconductors as well as in insulators.
Insulators are not appropriate due to the high value of the band gap which demands high energy photons to create the appropriate excitons for promoting both the reactions. The available photon sources for this energy gap are expensive and again require energy intensive methods. Hence insulators cannot be employed for the purpose of water splitting reaction.
Therefore, it is clear that semiconductors are alone suitable materials for the promotion of water splitting reaction.